1. Field of the Invention
The present invention relates to a new class Of antistatic additives based on solid copolymers of ethylene oxide having a high molecular weight. When used with plastics as an antistatic additive, the ethylene oxide copolymers do not significantly impair the physical properties of the plastic. The ethylene oxide copolymers are also stable at high processing temperatures and do not cause blooming of the finished product (oiliness, crazing, marblizing, and cracking). The present invention also relates to a method for treating polymeric materials with ethylene oxide copolymers to produce antistatic materials.
2. Prior Art
The retention and amplification of charges of static electricity on the surface of most plastics is well known. Plastic materials have a significant tendency to accumulate static electrical charges due to low electrical conductivity. A sudden discharge of static electrical charges in the form an electric arc is a fire hazard and exposes personnel handling the processed plastic to electrical shock. Furthermore, the presence of static electrical charges on sheets of thermoplastic film, for example, can cause the sheets to adhere to one another thus making their separation for further processing more difficult. Moreover, the presence of static electrical charges causes dust to adhere to items packaged in a plastic bag, for example, which may negate any sales appeal.
The increasing complexity and sensitivity of microelectronic devices makes the control of electrostatic discharge (ESD) of particular concern to the electronic industry. Only a few hundred volts can cause damage to sensitive devices and result in disappointing manufacturing yields, erratic errors, transient malfunctions, erased memory, etc. The need to control ESD requires the total assembly environment to be constructed of partially conductive materials. It also requires electrostatic protective packages, tote boxes, easings, and covers to store, ship, protect, or support electrical devices and equipment made from polymeric materials possessing antistatic properties. Additionally, accumulation of static electrical charges on electrical equipment like computers or vacuum cleaners or on finished goods such as carpet can cause the electrical equipment to fail or cause carpet to transfer accumulated static charges to the user during periods of low humidity.
To prevent static electrical charges from accumulating during manufacturing of the plastic, during forming of articles of manufacture, and during the useful life of the various articles of manufacture, it is well known to employ various types of antistatic agents. The antistatic agents can take the form of a coating which may be sprayed upon the manufactured sheets of plastic or the article of manufacture, for example, but this solution is often temporary.
Other attempts to overcome the static charges call for incorporating an antistatic agent into the plastic during processing in order to provide a more "permanent" antistatic characteristic to the manufactured plastic, articles of manufacture made from the plastic, etc.. However, the incorporation of antistatic agents into the various types of plastics have created other problems. For example, a majority of the antistatic agents cannot withstand high temperatures and they are destroyed or rendered useless with respect to their antistatic abilities during the hot temperature processing required in conventional molding or fabricating steps for forming articles of manufacture. This limits their applications to low-melting plastics, such as low density polyethylenes.
A majority of the antistatic agents are also either cationic or anionic. They tend to cause the degradation of plastics, particularly PVC and polyacetals, at hot processing temperatures resulting in discoloration or loss of physical properties. Other antistatic agents have significantly lower molecular weights than the polymers themselves. Generally, lower molecular weight antistatic agents possess undesirable lubricating properties and are difficult to incorporate into the polymer. Incorporation of the lower molecular weight antistatic agents into the polymers often reduce the moldability of the plastic because the antistatic agents bleed through to the surface of the plastic and frequently deposit a coating on the surface of the molds, thus destroying the surface finish on the articles of manufacture and reduce the dimensional stability when exposed to heat. In severe cases, the surface of the article of manufacture becomes quite oily and marblized. This is called blooming. Additionally, the lower molecular weight antistatic agents often tend to lose their antistatic capability due to evaporation, develop undesirable odors, and promote cracking or crazing on the surface of the article of manufacture.
One of the known lower molecular weight antistatic agents is a homopolymer or copolymer of ethylene oxide. Generally, the lower molecular weight polymers of ethylene oxide antistatic agents are subject to the above mentioned problems relative to lubricity, blooming, or less effective antistatic properties. Exemplary of the lower molecular weight antistatic agents comprising a homopolymer of ethylene oxide are Carbowax.RTM. by Union Carbide; Polyglycol.RTM. E by Dow Chemical, Pluracol.RTM. E by BASF Wyandotte, and U.S. Pat. Nos. 4,274,986; 3,745,116; and 3,625,915. Exemplary of the lower molecular weight antistatic agents comprising a block copolymer of ethylene oxide and propylene oxide are: Pluronic.RTM. by BASF Wyandotte; Voranol.RTM. by Dow Chemical; U.S. Pat. No. 3,686,120; Japanese Patents 49-23,246; 48-66,648; and 47-8,776; and German Patents 2,214,219 and 2,127,654. Exemplary of lower molecular weight antistatic agent comprising a random copolymer of ethylene oxide and propylene oxide are: Ucon.RTM. by Union Carbide; and U.S. Pat. No. 3,686,120 which is directed to external surface treatment.
Prior to the present invention, conventional thinking was that high molecular weight copolymers of ethylene oxide would significantly impair the physical properties of the plastic. Moreover, prior thinking was that a high molecular weight antistatic agent does not have good antistatic qualities and characteristics. In fact, prior art indicates that a molecular weight above about 20,000 to 25,000 is definitely undesirable. Exemplary of such prior art are U.S. Pat. Nos. 3,686,120 and 4,274,986.
The art of use high molecular weight ethylene oxide homopolymers have been described in U.K. Patent Application 2,139,230 and Japanese Patent 60-206,856. However, high molecular weight ethylene oxide homopolymers are difficult to incorporate into other polymeric materials probably due to the high crystallinity of homopolymers. Furthermore, homopolymers are uneffective antistatic agents when incorporated into polymeric materials at small amounts. When larger amounts of homopolymers are incorporated, the antistatic properties of polymeric materials are slightly improved, but the materials become extremely brittle.
The use of high molecular weight ethylene oxide polymers, including copolymers, for olefin polymers as antistatic additives has been disclosed in U.S. Pat. No. 3,425,981 to Puletti and Gardner. Poly(ethylene oxide) homopolymer is the preferred antistatic agent. The patent discloses that the antistatic agent is incorporated only into olefin polymer compositions. The patent fails to indicate the significant advantages of copolymers over homopolymers of ethylene oxide. Furthermore, the patent also fails to indicate the wide applications of antistatic copolymers to a variety of polymeric materials. The copolymers of ethylene oxide of this invention fail to show significant antistatic improvement when applied to any olefin polymer compositions such as polyethylene. When a copolymer of this invention comprising 85 weight percent of ethylene oxide and 15 weight percent of propylene oxide was incorporated at 10 weight parts per 100 weight parts of polyethylene, both low density polyethylene (Escorne.RTM. LD 405-49 from Exxon) and high density polyethylene (Amoco 3250B from Amoco) gave a surface resistivity great than 10.sup.16 ohm/sq, which is classified as an insulator. According to the teaching of U.S. Pat. No. 3,425,981, the copolymers of ethylene oxide of the present invention will be useless as antistatic additives.
Various copolymers of ethylene oxide having a high molecular weight are taught by the prior art. For example, German Patent 1,109,901 discloses a copolymer of ethylene oxide and propylene oxide which is formed by contacting the monomers with a coordination catalyst based on triethylaluminum (TEAL). Although the German patent does not disclose that these high molecular weight compounds can be used as antistatic agents, it does mention that the high molecular weight compounds can be utilized as thickeners and as a film base.
It is an objective of the present invention to provide an antistatic agent which is a solid, nonionic material preferably having a high molecular weight (greater than 20,000),-having a DSV of greater than 0.25,grams per millileter (g/ml) as determined on a solution made up with 0.25 grams of the polymer in 100 grams of toluene according to ASTM D2857, stable at high temperatures, inert toward and compatible with plastics, and yet do not cause blooming, do not develop undesirable odors, do not significantly impair physical properties and produces a "permanent" antistatic material with a surface resistivity in the order 10.sup.10 to 10.sup.13 ohms per square depending on the amount of antistat incorporated.
It is another objective of the present invention to produce a wide variety of antistatic polymeric materials incorporated in the ethylene oxide copolymers of this invention to minimize the problems caused by static electricity without impairing the physical properties or stability thereof.
It is another objective of the present invention to provide a process for treating a wide variety of polymeric materials with the ethylene oxide copolymers of this invention to produce antistatic polymeric materials.
These and other objectives and advantages of the invention will become apparent by reference to the following description.